Domestic uses of electricity and AC/DC

What you'll learn

This revision guide covers everything you need to know about domestic electricity for AQA GCSE Physics. You'll learn about mains electricity supply in the UK, the difference between alternating and direct current, how to wire a plug correctly, and how to calculate electrical power and energy in household appliances. These concepts are fundamental to understanding how electricity works in your home and frequently appear in exam questions.

Key terms and definitions

Alternating current (AC) — electric current that repeatedly reverses direction, typically 50 times per second (50 Hz) in UK mains supply

Direct current (DC) — electric current that flows in one constant direction only, supplied by cells and batteries

Live wire — the wire in a mains plug that alternates between high positive and negative voltage (approximately ±230 V), typically brown insulation

Neutral wire — the wire in a mains plug at or close to 0 V that completes the circuit, typically blue insulation

Earth wire — the safety wire in a mains plug at 0 V that prevents electric shock by providing a low-resistance path to the ground, typically green and yellow striped insulation

Power rating — the rate at which an electrical appliance transfers energy, measured in watts (W) or kilowatts (kW)

Frequency — the number of complete cycles of alternating current per second, measured in hertz (Hz); UK mains supply is 50 Hz

Potential difference — the voltage difference between two points in a circuit; UK mains supply provides approximately 230 V

Core concepts

Mains electricity supply in the UK

The UK mains electricity supply has specific characteristics that you must know:

• Voltage: approximately 230 V (sometimes stated as 230 V for precision)
• Frequency: 50 Hz
• Current type: alternating current (AC)

The frequency of 50 Hz means the current changes direction 50 times every second. This is different from battery-powered devices which use direct current (DC) where electrons flow in one constant direction.

Mains electricity is generated at power stations and distributed through the National Grid to homes, schools and businesses across the country. The use of AC rather than DC for mains supply makes it easier to transform voltages up and down using transformers, reducing energy losses during transmission.

Alternating current (AC) vs Direct current (DC)

Understanding the fundamental difference between AC and DC is essential:

Alternating current (AC):

• Current repeatedly reverses direction
• Voltage varies continuously between positive and negative values
• Used for mains electricity supply
• Graphically represented as a sine wave oscillating above and below zero
• Easier to transform to different voltages using transformers

Direct current (DC):

• Current flows in one constant direction
• Voltage remains constant (or decreases gradually as a battery discharges)
• Supplied by cells, batteries and solar panels
• Graphically represented as a horizontal straight line
• Used in electronic devices and portable equipment

Many household appliances contain transformers and rectifiers to convert AC mains supply to DC for use in electronic components. Mobile phone chargers, laptop adapters and LED power supplies all perform this conversion.

Three-pin plugs and cable insulation

A standard UK three-pin plug contains three wires, each with a specific function and colour code:

Live wire (brown):

• Carries the alternating voltage from the supply
• Alternates between approximately +325 V and -325 V (peak values)
• The stated 230 V is the root mean square (r.m.s.) value
• Most dangerous wire as it has high voltage

Neutral wire (blue):

• Completes the circuit
• At or close to 0 V
• Current flows in through live and out through neutral (or vice versa as AC alternates)

Earth wire (green and yellow stripes):

• Safety wire connected to the metal casing of appliances
• At 0 V under normal conditions
• If live wire touches the metal casing, current flows through earth wire to ground
• This causes a large current that blows the fuse or trips the circuit breaker
• Prevents the casing becoming live and causing electric shock

Wiring a three-pin plug correctly

You must be able to identify correct plug wiring:

1. Cable grip holds the outer insulation firmly to prevent wires being pulled loose
2. Live wire (brown) connects to the fuse and right-hand pin when viewing the plug face-on
3. Neutral wire (blue) connects to the left-hand pin
4. Earth wire (green/yellow) connects to the longest pin at the top
5. Wires must be firmly secured with no bare wire visible except in the terminal
6. Correct fuse rating must be fitted (see below)

The outer insulation protects all three wires. Each individual wire has its own colour-coded insulation to prevent short circuits. The pins are made of brass (a good electrical conductor) and the case is made from plastic, rubber or other insulating materials.

Fuses and circuit breakers

Fuses and circuit breakers are safety devices that disconnect the circuit if excessive current flows:

Fuses:

• Contain a thin wire that heats up and melts if current exceeds the rating
• Must be replaced after they "blow"
• Common ratings: 3 A, 5 A, 13 A
• Should be rated just above the normal operating current of the appliance

Circuit breakers:

• Electromagnetic switches that open (trip) when current exceeds a set value
• Can be reset by flicking a switch
• More convenient than fuses but more expensive
• React faster than fuses

Choosing the correct fuse:

To select the appropriate fuse, calculate the normal operating current using:

$$I = \frac{P}{V}$$

Where:

• I = current in amperes (A)
• P = power in watts (W)
• V = potential difference in volts (V)

Then choose the fuse with the next highest standard rating above the calculated current. For example, if an appliance draws 4.5 A, use a 5 A fuse, not a 3 A or 13 A fuse.

Electrical power and energy calculations

Power is the rate of energy transfer. For electrical appliances:

$$P = VI$$

$$P = I^2R$$

Where:

• P = power in watts (W)
• V = potential difference in volts (V)
• I = current in amperes (A)
• R = resistance in ohms (Ω)

The power rating of an appliance tells you how much energy it transfers per second. A 2000 W kettle transfers 2000 J of energy every second.

Energy transferred can be calculated using:

$$E = Pt$$

$$E = VIt$$

Where:

• E = energy in joules (J)
• P = power in watts (W)
• t = time in seconds (s)
• V = potential difference in volts (V)
• I = current in amperes (A)

For domestic electricity bills, energy is measured in kilowatt-hours (kW h):

$$\text{Energy (kW h)} = \text{Power (kW)} \times \text{Time (hours)}$$

One kilowatt-hour is the energy transferred by a 1 kW appliance in 1 hour.

To convert between joules and kilowatt-hours:

• 1 kW h = 3,600,000 J (or 3.6 MJ)

Cost of electricity:

$$\text{Cost} = \text{Energy (kW h)} \times \text{Cost per kW h}$$

Typical UK domestic electricity costs around 30p per kW h (though this varies).

Appliances and power ratings

Different household appliances have different power ratings reflecting their energy transfer rates:

High power appliances (typically 1000-3000 W):

• Electric kettles (2000-3000 W)
• Electric ovens (2000-3000 W)
• Immersion heaters (3000 W)
• Washing machines (2000-2500 W)
• Tumble dryers (2000-3000 W)

Medium power appliances (typically 100-1000 W):

• Vacuum cleaners (500-1000 W)
• Microwave ovens (700-1000 W)
• Televisions (100-400 W)
• Desktop computers (200-500 W)

Low power appliances (typically under 100 W):

• LED light bulbs (5-15 W)
• Mobile phone chargers (5-10 W)
• Laptop chargers (45-90 W)

High power appliances require thicker cables with lower resistance to carry larger currents safely without overheating.

Worked examples

Example 1: Calculating current and selecting a fuse

Question: A 2300 W electric kettle operates from the 230 V mains supply. Calculate the current flowing through the kettle and select an appropriate fuse from the following ratings: 3 A, 5 A, 13 A. (3 marks)

Solution:

• Use the equation: I = P ÷ V (1 mark)
• I = 2300 ÷ 230 = 10 A (1 mark)
• Select the 13 A fuse (the next rating above 10 A) (1 mark)

Mark scheme notes: Students must show the equation, perform the calculation correctly, and choose the next highest fuse rating. Choosing 5 A would be incorrect as it would blow during normal operation.

Example 2: Energy and cost calculation

Question: A family uses a 150 W television for 4 hours each day for 30 days. Electricity costs 28p per kW h.

a) Calculate the energy used in kW h. (2 marks)

b) Calculate the total cost. (2 marks)

Solution:

a)

• Total time = 4 × 30 = 120 hours
• Power = 150 W = 0.15 kW (1 mark)
• Energy = 0.15 × 120 = 18 kW h (1 mark)

b)

• Cost = energy × cost per kW h (1 mark)
• Cost = 18 × 28 = 504p = £5.04 (1 mark)

Mark scheme notes: Ensure power is converted to kW before calculating kW h. Show working clearly for method marks even if the final answer is incorrect.

Example 3: AC vs DC identification

Question: The diagram shows two oscilloscope traces of voltage against time for two power supplies.

[Trace A shows a sine wave oscillating symmetrically above and below the horizontal axis]

[Trace B shows a horizontal straight line above the horizontal axis]

a) State which trace, A or B, shows alternating current. (1 mark)

b) Explain your answer. (2 marks)

Solution:

a) Trace A (1 mark)

b) The voltage repeatedly changes from positive to negative values / the current reverses direction repeatedly (1 mark). Trace B shows constant voltage / direct current which does not reverse direction (1 mark).

Mark scheme notes: The explanation must reference the changing direction or reversing nature of AC. Simply stating "it goes up and down" is insufficient for full marks.

Common mistakes and how to avoid them

• Confusing live and neutral wire colours: Remember "BRown = BRight Right" (live is brown and connects to right pin). Blue is neutral, left pin. Don't use old colour codes (red/black) — these haven't been used since 2006.

• Choosing a fuse that's too large: Always calculate the normal operating current first, then select the next highest standard rating. A 13 A fuse in a 3 A appliance won't blow if a fault develops, creating a fire hazard.

• Forgetting to convert watts to kilowatts: When calculating energy in kW h, power must be in kW. Divide watts by 1000. A common error is calculating 2000 W × 3 hours = 6000 kW h instead of 6 kW h.

• Mixing up AC and DC characteristics: AC reverses direction (sine wave graph), DC flows in one direction (straight line graph). Don't state that AC "varies in magnitude" without mentioning direction reversal.

• Incorrect energy unit conversions: 1 kW h = 3,600,000 J (not 1000 J). To convert, remember: 1 kW = 1000 W, and 1 hour = 3600 seconds, so 1 kW h = 1000 × 3600 = 3,600,000 J.

• Stating the earth wire carries current during normal operation: The earth wire only carries current during a fault. Under normal conditions it's at 0 V and no current flows through it.

Exam technique for "Domestic uses of electricity and AC/DC"

• "Calculate" questions: Always write the equation first (using symbols), then substitute values with units, then calculate the answer with correct units. This ensures method marks even if your arithmetic is wrong. For 3-mark calculations, expect: 1 mark for equation, 1 mark for substitution, 1 mark for answer.

• Fuse selection questions: Never just give the fuse rating. Show your current calculation using I = P/V, then explicitly state you're choosing the next highest standard rating. This typically earns 2-3 marks compared to 0 marks for just stating "13 A fuse."

• "Explain" or "Describe" safety features: For earth wire questions, explain the sequence: fault occurs → live touches case → current flows to earth → large current → fuse blows → circuit disconnected → prevents shock. Each step can earn a mark.

• Graph interpretation: When identifying AC from oscilloscope traces, use precise terminology: "current/voltage repeatedly reverses direction" or "alternates between positive and negative values" scores marks. Vague descriptions like "goes up and down" often don't.

Quick revision summary

UK mains electricity is AC at 230 V and 50 Hz. Three-pin plugs have brown live wire (high voltage), blue neutral (0 V), and green/yellow earth (safety). Fuses and circuit breakers protect against excessive current. Calculate current using I = P/V to select appropriate fuses. Power (P = VI) measures energy transfer rate. Energy in kW h = power in kW × time in hours. Cost = energy × price per unit. AC reverses direction; DC flows constantly in one direction. Always show calculations clearly and use correct units.